1. Field of Invention
This invention relates generally to laser welding techniques for joining two parts together and more particularly to a technique in which the contacting faces of the parts define an interfacial region, one of the parts being transparent to the laser beam which is directed toward and focused on the region, the face of the other part having light disruptive characteristics producing multiple internal reflections in the region which trap the energy of the beam to generate sufficient heat to cause the parts to superficially melt and fuse.
2. Status of Prior Art:
In a welding process, two parts or pieces are joined together by applying heat to their adjacent surfaces to melt and fuse them. The heat for this purpose may be derived from various sources such as an arc, a gas flame, or from a laser beam, the latter being the concern of the present invention.
In carrying out welding, it is often the practice to interpose a filler or brazing material between the surfaces of the parts being joined. The filler has a lower melting point than that of the parts so that when heat is applied, the filler is melted to solder the parts together. This category of welding is commonly known as braze welding.
Laser beam welding is a well known technique, for a high power laser beam is capable of generating sufficient heat to melt metals and other materials having a high fusion temperature. Lasers in current use are mainly of the Nd:YAG, C0.sub.2, and Argon type. In practice, the laser beam may be pulsed or continuous.
Laser light is usually more intense, more monochromatic and more highly collimated than light from ordinary sources such as tungsten-filament lamps. The intensity of laser light can be extremely high. Thus power densities of over 1000 MW/cm.sup.2 are obtainable to produce a beam capable of cutting through and vaporizing solid materials. Lasers fall into four basic categories: solid state-optically pumped; liquid dye; semiconductor; and gas. Together, these four laser types cover the spectral region extending from ultraviolet to infrared.
The present invention is of particular value in connection with the manufacture of pressure transducers, such as the differential-pressure transducer disclosed in the Kazahaya patent 4,754,365. In a transducer of this type, a diaphragm made of glass or other transparent material is joined at its periphery to the flat face of a ceramic body having a nest indented therein to receive the diaphragm when it deflects in response to pressure.
One known technique to seal the periphery of the glass diaphragm to the body of the transducer is by means of a glass or ceramic frit. But with this technique, it is difficult to control the thickness of the resultant seal with high accuracy, so that the diaphragm is properly positioned.
The prior patent to Barnes et al., U.S. Pat. No. 4,424,435, discloses a laser braze welding technique in which a part to be welded to another component is transparent to radiation at the wavelength of the laser beam. A layer of brazing material is placed in the interface between the two parts, and the beam is directed through the transparent part toward this layer to cause it to melt and thereby solder the parts
The van Bennekom et al. U.S. Pat. No. 4,705,933 uses a laser braze welding technique for joining a transparent diamond component to metal. A layer of brazing material is heated by the laser beam which is focused through the diamond. The brazing material is a titanium-copper-silver eutectic having a brazing temperature of about 930.degree. C.
Because the laser braze welding techniques disclosed in the above identified patents make use of a filler, they suffer from the same problems which are encountered when using a glass or ceramic frit, for the melted filler which is interposed between the contacting surfaces of the parts usually is not of uniform thickness.
Also known for carrying out laser beam welding is a technique which uses no brazing filler for this purpose. Thus the Osborne U.S. Pat. No. 4,069,080 focuses a laser beam onto the contact region between two transparent thermoplastic sheets to weld them together. Since such materials have low melting points, the required laser beam energy is of relatively low power.
Also employing a braze-free laser beam welding technique is the U.S. Pat. 4,636,609 to Nakomoto in which two resin parts are welded together, one being transparent and the other having light absorbing properties. By directing the laser beam through the first part onto the light absorbing part, the heat developed at the interface is sufficient to melt and fuse the parts together.
The practical drawback to braze-free laser beam welding is that when the parts to be joined are not thermoplastic resins but are made of materials having high fusion temperatures, the amount of heat generated at the interface of the parts is insufficient to effect fusion even with high powered lasers. Thus if the two parts to be welded together are both more or less permeable to radiation produced by the laser, very little heat will be generated at the interface. And while the presence of an energy absorbing brazing filler at the interface would effect welding, the seal produced by this filler would not be of controlled thickness, and in many applications this would not be acceptable.
While the present invention will be described hereinafter mainly in connection with welding a diaphragm to the body of a pressure transducer, it is to be understood that this is by way of example only; for the laser welding technique has many other practical applications.